1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, and more particularly, to a multilayer ceramic substrate including a conductive via of a dual-layer structure capable of preventing loss in electrical conductivity and signal.
2. Description of the Related Art
A low temperature co-fired ceramics (LTCC) is highlighted as a packaging material of monolithic microwave integrated circuits (MMIC) in a radio frequency (mm wave) band of 30 GHz or more due to low dielectric loss and electrode loss. Also, a pure silver (Ag) paste is employed as an electrode material for wires to assure very low conductive loss. Moreover, a dielectric material has a loss coefficient of 0.2% for less which is at least 100 times lower than a printed circuit board (PCB), and thus exhibits superb characteristics in a radio frequency (mm wave) band.
However, a via connecting inter-layer wires should utilize an Ag electrode containing a great amount of glass in place of pure Ag to ensure matching of a sintering shrinkage ratio. Accordingly, this reduces electrical conductivity and attenuates an electrical signal to degrade characteristics of a final package.
In order to overcome this problem, studies have been vigorously conducted to match a sintering behavior of the Ag via electrode with LTCC chiefly by adding a minimum amount of predetermined additive to a pure metal. However, an effort to match the two materials without glass in a constrained sintering LTCC process has not been successful.
FIG. 1A is a graph illustrating a sintering shrinkage initiation period of pure Ag and LTCC according to the prior art. FIG. 1B is a horizontal cross-sectional view illustrating a via formed in a multilayer ceramic substrate according to the prior art. FIG. 1C is a graph illustrating a change in electrical conductivity of a via with respect to a higher content of glass in a via paste.
In a conventional manufacturing method of a multilayer ceramic substrate, a glass powder of 1 to 5 μm is mixed with an Ag powder of 2 to 3 μm to form a via paste for a constrained LTCC substrate and the via paste is filled in the via hole previously formed through metal mask printing. Several LTCC sheets each having the via filled are laminated into one and then sintered for 20 to 40 minutes at a temperature of 850-900° C. to produce a substrate.
Here, in a case where the paste for the via is formed of only pure Ag, as shown in FIG. 1A, there is a difference in a sintering shrinkage initiation period between an LTCC as indicated with A and Ag as indicated with B. Therefore, after sintering, as shown in C of FIG. 1B, voids or cracks occur on a wall of the via.
To prevent this, informing the via paste, a great amount of glass is added to the Ag powder to ensure matching of the sintering shrinkage initiation period with the LTCC. This via paste allows a via to be free from voids or cracks, after sintering.
However, as shown in FIG. 1C, an increased content of glass leads to poorer electrical conductivity than in a case where the via is formed by using pure Ag. That is, as for the via paste having glass added thereto, the via has electrical conductivity that is much lower than pure Ag via due to the glass with very low electrical conductivity. Accordingly, an increased amount of glass added to ensure matching of the sintering shrinkage initiation period with the LTCC results in lower electrical conductivity than in a case where pure Ag is employed.